1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for fabricating the same, and more particularly, to an LCD device with a simplified fabrication process, enhanced yield, and enhanced brightness and a fabricating method thereof.
2. Description of the Background Art
In the recent information society, a display has become increasingly important as a visual information transmission media. To satisfy this demand, a display should have low power consumption, a thin profile, light weight, and superior picture quality. A liquid crystal display (LCD) device, which is a major product type within the flat panel display (FPD) market, not only satisfies these needs but also enables a mass production so that each kind of new product using the LCD device becomes fast commercialized. As a result, the LCD device is replacing the cathode ray tube (CRT) as the primary display technology.
The liquid crystal display (LCD) device displays an image by individually supplying a data signal to liquid crystal cells arranged in a matrix form according to image information, and by controlling an optical anisotropy of the liquid crystal cells. An active matrix (AM), a main driving method used in LCD technology, drives a liquid crystal of a pixel region using an amorphous silicon thin film transistor (a-Si TFT) as a switching device. The amorphous silicon thin film transistor technique was established by English LeComber in 1979, and commercialized as a 3-inch liquid crystal portable television in 1986. Recently, an LCD of more than 50-inches was developed.
However, a field effect mobility (˜1 cm2/Vsec) of the amorphous silicon thin film transistor has a limitation to be used in peripheral circuits requiring a fast operation more than 1 MHz. As a result, development of techniques to simultaneously form a pixel region and a driving circuit region on a glass substrate using polycrystalline silicon (poly-Si) having a field effect mobility greater than that of amorphous silicon is needed.
The polycrystalline silicon thin film transistor technique has been applied to a small type module such as a camcorder since a liquid crystal color television was developed in 1982. Also, the technique has a low photosensitivity and a high field effect mobility so that a driving circuit can be directly fabricated on a substrate.
Increased mobility can increase an operational frequency of the driving circuit region which determines a driving pixel number, thereby facilitating high minuteness of a display device. Also, a signal voltage of the pixel region has a decreased charging time which decreases distortion of a transmitting signal and increases picture quality. When compared with the amorphous silicon thin film transistor having a high driving voltage (˜25V), the polycrystalline silicon thin film transistor can be driven under 10V, thereby decreasing power consumption.
Hereinafter, a structure of a related art LCD device will be explained with reference to FIG. 1. FIG. 1 is a plane view showing a structure of a driving circuit integrated LCD device in which a driving circuit region is integrated on an array substrate.
As shown, the LCD device comprises a color filter substrate 5, an array substrate 10, and a liquid crystal layer (not shown) formed between the color filter substrate 5 and the array substrate 10. The array substrate 10 includes a pixel region 35 formed as unit pixels are arranged in a matrix form; and a driving circuit region 30 arranged at an outer periphery of the pixel region 35 having a data driving circuit region 31 and a gate driving circuit region 32. Although not shown, the pixel region 35 of the array substrate 10 includes a plurality of gate lines and data lines for defining a plurality of pixel regions by being arranged horizontally and vertically on the array substrate 10; a thin film transistor (TFT) formed at each intersection between the gate lines and the data lines as a switching device; and a pixel electrode formed at the pixel region. Here, the TFT is a switching device for applying a voltage to the pixel electrode using a field effect transistor (FET) to control a current flow by an electric field.
The driving circuit region 30 of the array substrate 10 is positioned at an outer periphery of the pixel region 35 more protruded than the color filter substrate 5. The data driving circuit region 31 is positioned at a long side of the array substrate 10, and the gate driving circuit region 32 is positioned at a short side of the array substrate 10. The data driving circuit region 31 and the gate driving circuit region 32 use a thin film transistor having a complementary metal oxide semiconductor (CMOS) structure that is an inverter to properly output an input signal. The CMOS is an integrated circuit having an MOS structure used at a driving circuit region TFT that requires a high speed signal processing. The CMOS requires both an n-channel TFT and a p-channel TFT, and has speed and density characteristics corresponding to an intermediate level of an NMOS and a PMOS.
The gate driving circuit region 32 and the data driving circuit region 31 supply a scan signal and a data signal to the pixel electrode through the gate line and the data line, respectively, and are connected to an external signal input port (not shown). The gate driving circuit region 32 and the data driving circuit region 31 control an external signal input through the external signal input port thus to the external sync to the respective pixel electrodes.
A color filter (not shown) for implementing colors and a common electrode (not shown) facing the pixel electrode formed at the array substrate 10 are formed at the pixel region 35 of the color filter substrate 5. The color filter substrate 5 and the array substrate 10 are provided with a cell gap to be separated from each other by a spacer (not shown), and are attached to each other by a seal pattern (not shown) formed at an outer periphery of the pixel region 35 to constitute a unit LC panel. The color filter substrate 5 and the array substrate 10 are attached to each other by a bonding key formed at the color filter substrate 5 or the array substrate 10.
Since the driving circuit integrated LCD device uses a polycrystalline silicon TFT, an excellent device characteristic and a high picture quality are obtained. Accordingly, a high minuteness is implemented and power consumption is decreased.
However, the driving circuit integrated LCD device having an n-channel TFT and a p-channel TFT on the same substrate requires more complicated fabrication process than an amorphous silicon TFT LCD having only a single type channel. That is, to fabricate the array substrate including the polycrystalline silicon TFT, a large number of photolithography processes are required.
The photolithography processes form a desired pattern on a substrate by transferring a pattern from a mask to the substrate, and includes a plurality of processes such as a photoresist depositing process, an exposing process, a developing process, etc. Accordingly, the photolithography process degrades a production yield, and has a high probability of defects in the thin film transistor. Especially, since a mask designed to form a pattern is very expensive, a fabrication cost for an LCD device is increased when the number of masks is increased.